Currently absolute or direct reading encoders that can convert rotary or linear motion into a digital word representing absolute angular position are well known and readily available. They have the valuable property that the encoder operating power can be removed without loss of the record of absolute position since it is carried by the encoder itself. In providing this function, absolute encoders require a great many parallel scales, the exact number depending upon the range and accuracy requirements. This large number of scales in the electro-optical reading heads for absolute encoders makes them more delicate in field applications as well as somewhat less reliable. In many applications, the ruggedness and reliability are more important features than the inherent memory capability of the absolute encoder.
Incremental encoders which provide an output indication of each elemental change in angle or position are known in combination with circuitry which senses amount and direction of motion and accumulates these indicia in order to provide an indication of absolute position. Typically, a zero reference is also provided with such a system to reset the circuitry at a predetermined angular or linear position to prevent perpetuation of errors.